(1) Field of the Invention
The present invention relates to a system and method for measuring a sound velocity profile in a medium and particularly, to a system and method that uses an expendable vehicle to measure a sound velocity profile in a body of water surrounding a submarine or other water vessel.
(2) Description of the Prior Art
Underwater sonic equipment has many uses including, for example, fish and depth finding as well as sophisticated ranging and navigating. This equipment typically transmits a sonic signal from an underwater vessel, such as a submarine, and detects the return of an echo signal. Calculations based upon the elapse of time between the transmission of a sonic signal and the return of an echo makes it possible to ascertain the distance between the sonic transmitter and the object reflecting the sonic energy. Thus, knowledge of the sound velocity in the water between the submarine and the target is important to accurately interpret the acoustic data received from the sonar.
The sound velocity in water varies with localized conditions, such as water temperature, density, depth, salinity, and other factors. The sound velocity should therefore be measured at numerous points between the submarine or other such vessel and the target in order to obtain an accurate sound velocity profile. The present devices and techniques for measuring sound velocity, however, have a number of disadvantages.
One technique includes determining the velocity of sound by measuring the water temperature, salinity and other factors at given depths and then empirically finding the velocity using charts, tables and other data. These techniques are inherently inaccurate and time consuming. Another limitation is that the sound velocity can only be determined in those regions in which the temperature and other conditions can be measured.
Attempts have been made to measure the velocity directly within the medium itself, for example, using unmanned underwater vehicles (UUVs). Although the sound can be measured directly at locations remote from the submarine or other such vessel, the existing devices typically used to provide this direct measurement, e.g., conductivity-temperature-depth measuring devices (CDT), are expensive and require a relatively large, expensive vehicle for deployment. These devices must also be connected to the submarine or vessel using a fiber optic cable to relay the measured data back to the submarine. Because of the expensive equipment used in the UUV, the UUV must be recovered at the completion of its mission, a potentially dangerous and time consuming undertaking under combat conditions. Moreover, the submarine or other deployment vessel must be modified to allow it to recover the UUV.